A betaine in chemistry is any neutral chemical compound with a positively charged cationic functional group such as an ammonium ion or phosphonium ion (generally: onium ions) which bears no hydrogen atom and with a negatively charged functional group such as a carboxylate group which may not be adjacent to the cationic site. Historically the term was reserved for trimethylglycine only.
Phosphonium betaines are intermediates in the Wittig reaction.
In biological systems, many betaines serve as organic osmolytes, substances synthesised or taken up from the environment by cells for protection against osmotic stress, drought, high salinity or high temperature. Intracellular accumulation of betaines, non-perturbing to enzyme function, protein structure and membrane integrity, permits water retention in cells, thus protecting from the effects of dehydration. It is also a methyl donor of increasing significance.
Betaine (N,N,N-trimethylglycine) was named after its discovery in sugar beets (Beta vulgaris) in the 19th century. It is a small N-trimethylated amino acid, existing in zwitterionic form at neutral pH. This substance is often called ‘‘glycine betaine’’ to distinguish it from other betaines that are widely distributed in microorganisms, plants and animals.